Techniques for inspecting inside an inspection target in a non-destructive manner using X-ray transmission images have become essential techniques in various fields such as a field of medical services and a field of industrial-use nondestructive testing. Especially, X-ray image sensors which can directly capture X-ray transmission images as electronic data have become widely used because of their properties such as fast imaging, capability to assist in X-ray interpretation by using image processing, and capability to support moving images. Devices called an FPD (flat panel detector) are mainly used as such an X-ray image sensor.
Japanese Unexamined Patent Application Publication (JP-A) No. H04-206573 discloses a structure often used as the FPD (see FIG. 5 of the document) in these days. FIG. 13 depicts the structure, in which switching elements 700, photoelectric transducers 300 are arranged on substrate 200 in layers and a phosphor layer 600 for converting X-rays into light is further arranged on the layered structure with a passivation film 750 and a flattening film 760 inserted between the phosphor layer 600 and the layered structure. Each switching element 700 includes a gate electrode 710, a gate insulating film 720, an a-Si:H (hydrogenated amorphous silicon) film 730, and source and drain metals 740. Each photoelectric transducer 300 includes a lower electrode 310, an a-Si:H film 330 and an upper electrode 350. In the disclosed structure, electric signals according to the exposure intensities of X-rays are obtained by converting X rays which have passed through a specimen into light with the phosphor layer 600, converting the light into electric charges with the photoelectric transducers 300, and then outputting the electric charges from the structure by opening or closing the switching elements 700.
In FPDs widely used nowadays, a-Si:H TFTs are used as TFTs (thin-film transistors), which are switching elements, and a-Si:H photodiodes are used as photoelectric transducers. It should be noted that, although this example provides a structure that the switching elements, the photoelectric transducers and the phosphor layer are layered on the substrate in this order, FPDs can have another layered structure that a substrate, a phosphor layer, photoelectric transducers and switching elements are layered in this order as disclosed in FIG. 1 of JP-A No. H06-342078. In addition, there are FPDs employing a different technique for converting X-rays into electric signals, which do not use a phosphor layer. For example, JP-A No. S62-86855 (see FIG. 2 of the document) discloses a structure that uses a photoconductor layer which converts X-rays into the electric charges directly, and discloses a method that uses a layer of Bi12Ge20 as an example of the photoconductor layer. However, the direct type FPDs which convert X-rays into electric charges directly have a disadvantage of quantum efficiency being lower than the indirect type FPDs which use a phosphor layer. There are several types of FPD structure as described above. However, all of them have a structure in which at least switching elements and photoelectric transducers are formed in layers on a substrate.
During recent years, in the field of medical services, there are increasing demands on X-ray diagnosis devices to realize higher resolution and support fluoroscopy (taking moving or real-time images) on radiographic imaging. This is because higher resolution is essential for observing an affected part in more detail, and fluoroscopy is required for checking an optimal condition for radiographic imaging and finding out a proper timing for radiographic imaging. In order to increase the resolution of FPDs, there is a need to read signals at high speed from an increased number of photodetectors which have been prepared for realizing the high resolution. In order to support fluoroscopy, it is necessary to read the signals from all the photodetectors during a predetermined frame period. That is, it is necessity to read the signals at far more higher speed in order to allow an FPD having higher resolution to support fluoroscopy.
The main cause of the limitation of the signal read-out speed of FPDs is on-state resistance of the switching elements. In conventional FPDs, a-Si:H TFTs are used for the switching elements. The field-effect mobility of a-Si:H is as low as 1 cm2/Vs or less. This causes high on-state resistance of the a-Si:H TFTs. Alternatively, there are poly-Si (polycrystalline silicon) TFTs, which are switching elements that can be formed on a large-sized substrate. The field-effect mobility of poly-Si is reported to have as high as more than 100 cm2/Vs. Thus, the on-state resistance of the poly-Si TFTs is very small.
However, poly-Si TFTs have another problem that their threshold voltages vary largely. Such variations in threshold voltages cause variations in the signal electric charges in FPDs, causing FPN (fixed pattern noise). The FPN can be corrected by modifying a signal readout circuit, but it causes other problems about narrow dynamic range and/or high cost of the signal readout circuit. Variations in the threshold voltages of poly-Si TFTs that lead to the FPN are an essential problem resulting from the crystal structure of poly-Si TFTs being a polycrystal, and thus the poly-Si TFTs are hardly used for FPDs.
In recent years, amorphous oxide semiconductor has been developed rapidly. In—Ga—Zn—O is the representative case. The field-effect mobility of amorphous oxide semiconductor is about 10 cm2/Vs to 20 cm2/Vs and is an order-of-magnitude larger than that of a-Si:H or more. Therefore, the on-state resistance of a TFT formed by the amorphous oxide semiconductor is an order of magnitude smaller than that of an a-Si:H TFT or less. Further, since its structure is amorphous, the problem that threshold voltages vary largely does not arise like poly-Si TFTs do.
JP-A No. 2009-71057 discloses an example of applying amorphous oxide semiconductor to an image sensor (see FIG. 3 of the document). The image sensor described therein has a structure in which a plurality of photodetectors are arranged on a substrate in layers, where each photodetector includes a switching element and a photoelectric conversion section having sensitivity to a specific wavelength. A unit structure of the photodetector is illustrated in FIG. 14. This structure is formed by arranging a TFT as a switching element 100 and a photoelectric transducer 800 on the substrate 200 in layers, with passivation film 150 inserted between them. The TFT is composed of a gate electrode 110, a gate insulating film 120, an amorphous oxide semiconductor film 130 and source and drain electrodes 140. The photoelectric transducer 800 is composed of a lower electrode 810, a photoelectric conversion film 820 and an upper electrode 830. If amorphous oxide semiconductor is applied to an image sensor as in this case, it may be possible to achieve high speed signal read-out.
Although amorphous oxide semiconductor has excellent characteristics, it has problems of being unable to control off-state current depending on the manufacture method. The cause of this problem is that control of oxygen deficiency (oxygen holes) of the amorphous oxide semiconductor is difficult as described in JP-A No. 2008-42088 (see paragraph 0006) and JP-A No. 2008-60419 (see paragraph 0005 of the document). As a countermeasure, JP-A No. 2008-42088 discloses a structure that two insulating layers that are laminated so as to sandwich an amorphous oxide semiconductor layer, and a technique of making oxygen hole concentration of either one of two interfaces of the insulating layers in contact with the amorphous oxide semiconductor layer smaller than oxygen hole concentration in an amorphous oxide semiconductor film (see claim 10 of the document). In addition, JP-A No. 2008-60419 discloses a structure that a first insulating layer and a second insulating layer are laminated on an amorphous oxide semiconductor film, and a method of oxidizing the first insulating layer before laminating the second insulating layer thereon (see claim 1 of the document).
However, in cases where amorphous oxide semiconductor is applied to TFTs in an FPD having a structure where TFTs, which are switching elements, are formed on a substrate, and then photoelectric transducers made of a-Si:H are formed as illustrated in FIG. 13, the inventors of the present application found that characteristics of the TFTs vary largely and thus the image sensor cannot be operated stably even if manufacturing methods described in, for example, JP-A Nos. 2008-42088 and 2008-60419 are applied to the FPD. The result was the same in cases where photoelectric transducers made of a-Si:H are formed on the substrate, and then TFTs made of amorphous oxide semiconductor are formed thereon.
As a result of a further analysis of the inventors, the cause of the above matter has been found as follows. That is, in a case where a film of amorphous oxide semiconductor is formed on a substrate, and then an a-Si:H film is formed thereon, hydrogen contained in raw material gas of the a-Si:H film permeates the film of amorphous oxide semiconductor, which causes the characteristics deterioration of the TFTs. In another case where an a-Si:H film is formed and then a film of amorphous oxide semiconductor is formed thereon, hydrogen is separated from the a-Si:H film, which forms photoelectric transducers, as a result of the temperature rise caused upon a situation, such as a situation that the film of amorphous oxide semiconductor is formed and a situation that an insulating film to be laminated on the TFTs is formed, and the hydrogen permeates up to the film of the amorphous oxide semiconductor, which also causes the characteristics deterioration of the TFTs.
The present invention seeks to solve the problems.